Solid state imaging devices, e.g., CCD, CMOS, and others, may include a wafer-level lens or series of wafer-level lenses to direct incoming light onto a focal plane array of pixels. Each one of the pixels includes a photosensor, for example, a photogate, photoconductor, or photodiode, overlying a substrate for accumulating photo-generated charge in an underlying portion of the substrate. The charge generated by the pixels in the pixel array is then read out and processed to form an image.
FIG. 1 is a diagram of a portion of an imager module 100 having a wafer-level focusing lens structure attached to a substrate 110 and a pixel array 120, the latter of which is part of an imager die. It should be understood that the focusing lens may be a simple or compound lens of varying shape and that only the substrate 110 on which the lens sits is shown in FIG. 1. One end of the substrate 110 is spaced at a distance x from the pixel array 120. The pixel array 120 includes a plurality of microlenses 122, arranged above a respective pixel cell 124, that focus incoming light onto the photosensor of the respective pixel cells 124.
Conventional imager modules 100, such as the one in FIG. 1, include a gap 130 between the illustrated end of the substrate 110 and the pixel array 120 that is filled with air. The index of refraction of air (nair=1) is lower than the index of refraction of the substrate 110, which may be made of glass. The gap 130 and the difference in the indexes of refraction in a conventional imaging device cause the light rays 140a, 140b, 140c to be refracted at the interface (shown by arrow A) between the substrate 110 and the air gap 130.
Light rays 140a, 140b, 140c are generally focused by the focusing lens structure into a conical bundle of light rays 140. The light ray 140a in the center of the bundle of light rays 140 is known as the chief ray and the angle of the chief ray is known as the chief ray angle. The chief ray angle is measured in relation to the normal of the planar surface 156 of the image sensor 120, with an angle of zero degrees being perpendicular to the planar surface 156. The materials, shapes, and distance x from the pixel array 120 of a focusing lens structure are generally selected to optimally focus the bundle of light 140 having its chief ray angle 140a at zero degrees.
However, as shown in FIG. 2, if light rays 140 enter the imager module 100 and pass through the substrate 110 at a chief ray angle that is sufficiently oblique or acute, light rays 140a, 140b, 140c exiting the substrate 110 at the interface between the substrate 110 and the air gap 130 will be refracted outwards due to the difference between the index of refraction of the air gap 130 and the index of refraction of the substrate 110 so that the light rays 140a, 140b, 140c may miss the pixel array 120 entirely, are not focused properly, or enter the image sensor at a too large angle. In some instances, light rays may be partially or totally internally reflected as represented by light ray 140e. The loss of light and poor focusability of light rays 140 having a high chief ray angle will negatively affect the quality of an image generated by the pixel array 120.
What is needed is a system and method by which light rays having a high chief ray angle are not refracted away from a pixel array of an imaging device at the transition from the lens to the pixel array.